1. Field of the Invention
The present invention generally relates to lighting devices, and more particularly to a lighting device providing individual adjustment of the brightness of red, green, and blue light emitting diodes (LEDs) for full-color illuminating and presentation effects on the lamp shade or screen by controlling the amount of current flowing through the LEDs.
2. The Prior Arts
Currently, most of the LED-based color lamps available in the market provide various colored light by individually turning on and off the red, green, and blue LEDs of the lamp. For example, a control circuit of the lamp provides a 3-bit digital signal with each bit controlling whether to turn on (1) or off (0) the red, green, and blue LEDs, respectively. The lamp is therefore able to generate light of seven different colors, including red (001), green (010), blue (100), red plus green (011), green plus blue (110), blue plus red (101), and red plus green plus blue (111). In order to generate light of more different colors, the brightness of the individual red, green, blue LEDs has to be adjustable, instead of having only ‘on’ and ‘off’ two choices, so as to produce more color combinations. However, adjusting the brightness of LEDs by voltage is difficult due to the specific semiconductor characteristics of LEDs. It is well known that, when the LED is forward-biased, a small variation in the voltage applied would incur significantly different amount of current conducting through the LED.
Recently, some techniques for adjusting the brightness of LEDs are disclosed. For example, in the R.O.C. Patent Application No. 89127119, an analog control signal is converted to a digital signal with 256 levels. The digital signal is then applied to a pulse width modulator (PWM) whose output pulses conduct the LED periodically. As the LED is “flashed” very fast, the light from the LED is perceived as continuous and steady due to the visual persistence of human eyes. The digital signal alters the duty cycle of the pulses output by the PWM and, by increasing or decreasing the duty cycle of the pulses, the LED is able to deliver light of various brightness. However, when the brightness of the LED is reduced (i.e., low duty cycle), an observer would perceive flickers as the 1/16-second maximum duration of human visual persistence is exceeded. A lamp based on the PWM-based approach therefore can only provide stable lighting in the medium-to-high brightness range and delivers only a small number of color combinations. In another technique disclosed in R.O.C. Patent Application No. 93102287, a PWM is also used to vary the brightness of two sets of LEDs. This technique would suffer the same drawbacks of the previous one.
In addition, the aforementioned prior arts are for illumination purpose and therefore several tens of LEDs have to be adopted as the light source. Currently, the efficiency of LEDs is compatible to lamps using tungsten filament, in which around 80% of the power is converted into heat and wasted. Therefore, when a large number of LEDs are used, complicated heat dissipation mechanism has to be designed and employed, causing significant cost increase and reducing the applicability of the LED-based lamps.